1. Field of the Invention
The present invention relates to a method for color measurement on printing materials produced by a printing press, using at least a first color measuring device installed in the printing press and at least a second color measuring device.
In order to assess the quality of printing materials produced, the use of color measuring devices is known. By means of the color measuring devices, differences in the color between printing original and printing material produced can be detected even in a range which the human eye does not perceive clearly. In addition, by means of color measuring devices, the subjective impression of an observer can be avoided and automatic color control can be implemented. At present, calorimetric and densitometric color measuring devices are used. Of these there are three types, firstly so-called in-line measuring devices, which is to say color measuring devices which are arranged within the printing press and measure the color of printing materials continuously during continuous printing operation. Such measurements are not normally made in the printed image itself but on print control strips applied to the edge of the printing material. Furthermore, there are separate color measuring devices in the form of handheld measuring devices or measuring tables, on which the finally produced printing materials are placed and registered by a color measuring device. The color measuring devices can be connected to a control computer of the printing press, in order to carry out color control by means of a desired value-actual value comparison of the color on the printing original and the printing materials measured. The color control then acts on the inking units in the offset printing press and in this way attempts to equate the color of the finally produced printing materials to the printing original.
Commonly assigned U.S. Pat. No. 5,835,626 and its counterpart European published patent application EP 0 741 029 A2 disclose a method for open-loop or closed-loop control of the color during the printing process in printing presses, wherein measured color data is obtained continuously from printed images created and is used for the control and regulation (i.e., closed-loop control) operations to influence the color on the printing materials. If a specific tolerance limit is violated, control or regulation of the inking units in the printing press is carried out. However, a wait is not made until the tolerance limits are violated; instead a trend estimation is performed in order to extrapolate the development over time of the measured color data registered in the future. As soon as the result of the trend estimation is that the extrapolation of the trend would lead to a violation of the tolerance limits, a preventative intervention is made in the color control or color regulation. In this way, the predefined tolerance limits are not reached at all. The disadvantage with such a system is, however, that, on account of the continuously determined measured color data, a correspondingly high computing power is needed for the trend estimation in order to be able to react in good time to the trend that is manifested. Furthermore, the method according to U.S. Pat. No. 5,835,626 and EP 0 741 029 A2 is suitable only for the continuous printing process when the state of the color is already extensively stable. During the setup phase, however, the color is subject to relatively large fluctuations, so that here a trend estimation as in U.S. Pat. No. 5,835,626 and EP 0 741 029 A2 is not able to supply any criteria for reaching a stable state.